The conventional internal combustion engine is one where the cylinders, either in-line or in a V-block, for instance, have the cylinder connecting rods connected to a crank shaft and the crank shaft is rotatably driven by the combustion of the fuel mixture within the cylinders. The typical combustion cycle includes intake of an air-fuel mixture into the cylinder, compression of the air-fuel mixture by the piston, combustion which causes a rapid expansion of the gases within the cylinder to drive the piston and perform work, and the subsequent exhaust stroke evacuating the products of combustion. In a four stroke crank-type engine, the power or expansion stroke occurs once in each 720.degree. of rotation of the crank shaft.
This conventional internal combustion engine also requires an intake and exhaust valve for each cylinder which must be timed to open and close in synchronization with the cycle of the pistons. The valves in a conventional internal combustion engine are poppet valves which have a stem and a mushroom shaped head with edges seating on the periphery of a valve opening and which are opened and closed by synchronized cams. Because the seating faces of the exhaust valves in internal combustion engines are subjected to extremely high temperatures they tend to burn, oxidize or provide a source of pre-ignition. Pre-ignition is frequently a source of damaging engine knock. Accordingly, it is necessary to cool the valve, limit operating temperature and/or maintain a reducing atmosphere during combustion. In a conventional engine this is accomplished by using an excess of fuel, i.e. a rich mixture, over that necessary to support the combustion process. This excess fuel is utilized as a coolant for the exhaust valve as well as insuring that there is no free oxygen at the end of the combustion process, which could oxidize the valves. Because excess fuel is supplied to the cylinders, all the fuel is not completely combusted and unburned hydrocarbons from the uncombusted fuel are exhausted through the exhaust valves and the exhaust manifold system rather than contributing to the output power. Because of this, the exhaust gas from the internal combustion engine pollutes the atmosphere excessively.
The use of the crank shaft in a conventional internal combustion engine causes a kinematic limitation to the motion of the piston. That is, the translation of the reciprocating motion of the piston to rotary motion by means of a crank causes the piston to reciprocate up and down in the cylinder in the characteristic crank-slider motion, which is a higher order, non-sinusoidal motion. This characteristic crank-slider motion cannot be conveniently altered and is symmetric for each stroke, because it is fixed by the geometry of a crank/connecting rod assembly. The crank-slider motion of the piston in a conventional internal combustion engine is disadvantagous for several reasons, including: 1) crank-slider motion generates higher inertial stresses than does pure sinusoidal motion, 2) crank-slider motion results in increased time at or near top dead center ("TDC"), increasing the likelihood of pre-ignition, 3) increased dwell time results in increased heat loss to the engine both before and after firing, and 4) the torque arm just after firing is small, under utilizing the high gas pressures and 5) the torque arm near the end of the the stroke when pressure is low, i.e. near bottom dead center ("BDC") is too small for effective capture of the motive force in this gas. Furthermore, the crank-slider motion does not closely match the heat and pressure conditions as a function of time that are created in the combustion chamber during the operation of the engine.
In spark ignition engines the longer the time period that the air-fuel mixture is compressed the greater likelihood there is of pre-ignition. Because the upward rise of the piston in a conventional engine is relatively slow near TDC, the compressed air-fuel mixture is at or near its maximum compression during a relatively long period of time prior to top dead center. For this reason, relatively low compression ratios and/or high octane fuels are required to prevent pre-ignition.
Immediately after passing top dead center and beginning its downward expansion stroke, the piston in a crank-type engine is also moving relatively slowly. In both spark-ignition engines and compression-ignition (i.e. Diesel) engines, the relatively slow motion of the piston near top dead center causes excessive heat loss because of the relatively long length of time that the hot combustion gases are in contact with the head and cylinder walls. Finally, the crank-slider motion of the piston near the end of its stroke, that is near bottom dead center, where the pressures are the lowest, makes it difficult to effectively utilize the available motive force in the gases, due to the pressures involved coupled with the short effective arm of the crank at this position. Thus, in conventional engines, the exhaust valve begins to open a significant number of degrees before bottom dead center, resulting in a significant loss of available energy of the combusted gases.
Furthermore, in a crank-type engine, the intake stroke of the piston in a four stroke engine is inherently the same length as the expansion stroke. Because of the increase in temperature and pressure caused by combustion, at the bottom of the expansion stroke (even if the exhaust valve were not to be opened until bottom dead center), the combusted gases will still be at a higher pressure than ambient. Thus, significant loss in available motive force in the combusted gases occurs when the exhaust valve opens and exhausts the higher than ambient pressure gas to ambient pressure. Various mechanisms combined with the crank/connecting rod system have been proposed to try to capture more of this available work through more complete expansion, but have not proven successful due to their cost and mechanical complexity. For example, the Atkinson mechanism provides a crank/connecting rod system with a longer expansion stroke than intake stroke, but at greatly increased mechanical complexity.
Moreover, the octane quality of commercially available fuels, which affects the permissible compression ratio, varies considerably. Making provision for variable compression ratio in the cylinders would allow the maximum permissible compression ratio for a given fuel, and hence highest efficiency for a given fuel. However, efforts to make internal combustion engines with variable compression ratios have not proven very successful in practice due to mechanical complexity. Thus, conventional internal combustion engines have non-adjustable compression ratios and engine manufacturers must design compression ratios to accept the poorest available fuel. This compromise results in an engine having a lower compression ratio than the optimum, and hence a lower efficiency than the optimum for an average fuel. Gasoline manufacturers sell "super" octane gasoline, therefore conventional engines designed for poor fuel derive no benefit from using these costly "super" fuels.
In an attempt to alleviate some of the difficulties of the crank-type internal combustion engine, various rotary engine designs have been proposed where the engine block housing the cylinders and pistons of the engine is directly coupled to the output shaft of the engine and the entire block, with the assembly of cylinders and pistons, rotates along with the output shaft. In one such rotary engine proposal, U.S. Pat. No. 4,023,536, each piston has a roller which rolls against the interior surface of a cam to translate the reciprocating motion of the piston to rotary motion of the engine block rotor, instead of by means of a crank and connecting rod as in a crank-type engine.
Although the use of a cam overcomes the inherent kinematic limitations of a crank mechanism, these rotary designs have not been entirely successful. In such rotary engine designs the cam acts directly upon the roller which is directly conected to the piston. Since it is the tangential (i.e. side) component of force from the cam surface which causes rotation of the engine block, and hence the useful power output, these forces can only be transmitted to the engine block in these designs by means of side forces on the piston against the cylinder walls. These side forces and friction contribute to excessive wear on the piston and cylinder in these prior art designs.
Furthermore, because the entire engine block and pistons rotate in a rotary engine, centrifugal force tends to throw the piston outward against the cam. These centrifugal forces are very large in magnitude, tend to increase wear on the cam surface and cam roller in prior art rotary engine designs thereby limiting engine speeds adversely.
In rotary engines, the engine block with the cylinders rotates within a housing. Because of this, cooling the cylinders has proven difficult in prior art designs, because delivering sufficient air or water to a rotating assembly of cylinders presents mechanical and sealing difficulties.
These and other problems have thus far prevented the the practical implementation of a rotary engine design.